The present invention relates to a wideband multichannel fiber laser used in WDM (Wavelength Division Multiplexing) optical communication systems, more particularly, to a wideband multichannel fiber laser whose wavelength range is expansible to increase the number of lasing channel, and whose channel output powers are equalized, in order to provide a useful multichannel laser source having a wide bandwidth.
A multichannel fiber laser is an apparatus for simultaneously outputting multiple laser lights having different wavelengths with uniform wavelength interval from one fiber laser apparatus. Such a fiber laser can be used as a light source for signal transmission in a WDM optical communication system and for characterization and evaluation of WDM devises and apparatuses.
A laser is composed of a gain medium, a pumping light source and a resonator, alternatively called a laser cavity. Among various materials that can be used as a gain medium of a fiber laser, an erbium-doped fiber (EDF) is most widely used. The EDF has also been used to make an optical fiber amplifier in an optical fiber communication system, known as an erbium-doped fiber amplifier (EDFA). A semiconductor laser, that is commonly known as a laser diode, having a wavelength of 980 nm or 1480 nm is used as a pumping light source and a WDM coupler is used to launch the pumping light into the EDF. A conventional directional coupler is used as an output coupler that extracts the laser output from a laser cavity. All the components including the WDM coupler, the EDF and the directional coupler are fusion-spliced or connected to form a laser cavity.
In EDF, small signal gain is established in proportion to launched pump power. When the small signal gain becomes equal to cavity loss, laser oscillation takes place at the wavelength of peak gain in spectral domain. Once the laser oscillation takes place, the laser output power is determined so that the saturated gain of the EDF may be equal to cavity loss. Consequently, the small signal gain at the lasing threshold is always the same as the saturated gain above the lasing threshold.
An optical filter is required when the laser oscillation at a specific wavelength is desired out of the wide gain bandwidth of EDF. If the optical filter passes-a single wavelength only, it is called a single channel filter and the laser containing this filter oscillates in a single wavelength or a single channel. If the optical filter passes multiple wavelengths, it is called a multichannel filter and the laser containing this filter may oscillate in multiple wavelengths or multiple channels. However, since the gain spectrum of EDF is dominated by homogeneous line broadening at room temperature, an erbium-doped fiber laser (EDFL) tends to oscillate in a single channel even though the multichannel filter is used. Particularly, if an optical isolator is inserted in a ring cavity to enable unidirectional laser operation, spatial hole burning effect does not appear in the gain medium, so that the laser is highly favorable to single longitudinal mode oscillation. Therefore, suppressing the tendency toward single mode oscillation is a crucial technology to obtain multichannel laser oscillation from EDFL s.
Gain bandwidth of EDF reaches xcx9c80 nm in 1550 nm wavelength range. If multichannel laser oscillation could be obtained with 100 GHz WDM channel spacing defined by ITU(International Telecommunications Union) from an EDFL, the maximum number of channel simultaneously available from one light source would be 100. Needless to say, all channels must satisfy the previously mentioned laser oscillation condition in order to realize such an efficient multichannel light source. For example, when the wavelength of the light selectively filtered by a multichannel filter is xcexi (i=1,2,3), the following equation 1 should be satisfied in logarithmic scale:
Gs(xcexi)=Lc(xcexi)xe2x80x83xe2x80x83(1),
where Gs and Lc represent saturated gain and cavity loss, respectively.
Generally, the cavity loss, Lc has little dependence on wavelength, which means Lc is almost a constant function of wavelength. The saturated gain, Gs, however, has an inherent spectral profile depending on both pump power and input signal power owing to saturation characteristics of a gain medium. Consequently, in an ordinary condition, the equation 1 cannot be satisfied at all wavelengths. In case of a homogeneously broadened gain medium such as EDF, if the equation 1 is first satisfied at the wavelength of peak gain and laser oscillation takes place at this wavelength, the gain profile remains unchanged even though the pump power increases, which results in single channel oscillation, and the equation 1 cannot be satisfied at the other wavelengths.
Therefore, to find out a way to make optical fiber lasers oscillate in multiple channels and to increase the number of useful channels is becoming important technology in the field of WDM optical communication.
There have been 4 major methods invented to obtain multichannel laser oscillation from optical fiber lasers.
The first method is, as shown in FIG. 1, to cool EDF down to extremely low temperature by using a cooling device such as liquid nitrogen in order to reduce the homogeneous linewidth down to 0.5 nm or less. When EDF has the homogeneous linewidth of less than 0.5 nm, the two lights whose wavelengths are separated by 0.5 nm or more can obtain independent gains from EDF, which results in simultaneous laser oscillation at the two wavelengths. A birefringence filter consisting of a polarization maintaining fiber(PMF) 5, a polarizer 3 and a polarization controller (PC) 3 was used as a multichannel filter. The laser was made to oscillate unidirectionally using an optical isolator 6. The laser output is obtained via a 10% coupler. This method has played an important role in analyzing the characteristics of EDFL s. However, this method has limited applicability since it uses a cooling device such as liquid nitrogen that is hard to maintain.
FIG. 2 shows a schematic of the second method of realizing multichannel fiber lasers. It uses a 1xc3x97N coupling device for branching a light into a plurality of optical paths, such as a multi-branch optical fiber coupler and an AWG(Arrayed Waveguide Grating) filter, and a Nxc3x971 coupling device for multiplexing the branched light. Each optical path between 1xc3x97N and Nxc3x971 coupler contains a piece of EDF and an optical tunable filter (TF) having different transmission wavelength so as to selectively pass and amplify the light of a specific wavelength defined by the filter. With this configuration, each light of the corresponding optical path is able to obtain independent optical gain, so that the laser as a whole may operate in multiple channels defined by TF s. This method allows the multichannel fiber laser to operate at room temperature and the wavelength stability and the output power of each channel can be controlled by the corresponding TF and variable optical attenuator (ATN), respectively, channel by channel. However, this method eventually makes the laser system complicated when the demand of channel number increases, because the addition of channel requires additional set of EDF, an optical tunable filter, and a variable optical attenuator.
The third method is, as shown in FIG. 3, to connect a number of single channel fiber lasers, 8A-8B, with different wavelengths serially. FIG. 3 shows a serially connected fiber laser only, but it is possible to connect fiber lasers parallel. An optical fiber DBR(Distributed Bragg Reflector) laser and an optical fiber DFB(Distributed Feed Back) laser may be used as the individual fiber laser. Both types of fiber lasers use FBG(Fiber Bragg Grating) technology. This method also allows the laser system to operate in room temperature, and has the advantage of simplicity in both concept and configuration. However, it requires an equipment for fabricating the FBG to make multichannel fiber lasers with specific channel spacing and channel number, so this method requires large initial investment.
The final method, as shown in FIG. 4, has a configuration similar to that of the first method, but it has a frequency shifter instead of the cooling device for EDF. The conception of this method originates from a spectrum sliced light source (SSLS) that is a multichannel filtered ASE(Amplified Spontaneous Emission) light generated by an EDFA. This method was attempted to amplify the SSLS several times via an optical fiber ring resonator with a gain medium in order to achieve high output power. In FIG. 4, the frequency shifter was inserted to prevent the fiber laser from oscillating in a single wavelength and thus to make it advantageous to multichannel oscillation. Since the fiber laser using this method is able to operate in stable multiple channels even when all the channels share the same gain media, EDFA1 and EDFA2, at room temperature, it has advantages in both configuration and efficiency.
Prior techniques related to increment of the number of channel in the described multichannel fiber lasers are disclosed hereafter.
In the multichannel fiber laser of the first method, cooling the EDF is itself the method of increasing the number of oscillation channel. Suppression of internal reflections of a laser cavity to eliminate undesired filtering effect and the use of polarization hole burning effect of EDF have been demonstrated. However, these are merely the methods to remove the elements disturbing multichannel laser oscillation at laboratory level. The method for obtaining the desired number of multichannel oscillation in the desired wavelength range in the viewpoint of laser system design has not been developed.
In the second method, the number of channel is determined by the number of optical paths of the used multi-branch optical fiber coupler or multiplexer. Therefore, the appropriate devices have to be carefully chosen according to initial laser system design. Once the devices are chosen, there is no option to change the number of channel and the wavelength range.
In the third method, additional connection of new fiber lasers of different wavelength is to increase the number of channel.
Finally, in the fourth method, there has been no special technique about channel number increment developed.
The objects of the present invention are to provide a way to increase the number of lasing channel by enlarging the oscillation wavelength region in multichannel fiber lasers, and at the same time, to provide a useful multichannel laser source with equalized output power for all channels.
The present invention provides a multichannel fiber laser in which all the laser channels share one or more gain offering means comprising: a gain offering means for providing optical gain; a gain equalizing means for enlarging laser oscillation wavelength range, increasing the number of laser channels, and equalizing the optical gain from the gain offering means and output powers of all laser channels; and a resonating means including the gain equalizing means for selecting light channels, adjusting loss, controlling state of polarization, and outputting light to outside of the fiber laser.
Also, the present invention provides a method for designing a filter for equalizing a gain in a multichannel fiber laser having a resonator, a gain medium and a filter for equalizing gain profile. The method comprises the steps of: a first step of measuring loss of a light having passed the resonator as a function of the wavelength of the light source; a second step of measuring a small signal gain per unit length having passed the gain medium as a function of the wavelength of the light source; and a third step of determining a length of the gain medium so that the small signal gain may be the same as or larger than the resonator loss at all wavelengths within range of interest at a threshold pump power.